1. Field of the Invention
The present invention relates to a developing device which develops an electrostatic latent image with use of two-component developer, and to an image forming apparatus having the developing device.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming images in the xerography process has a developing device by which an electrostatic latent image formed on a photoreceptor is developed. One example of the developing device is a developing device which uses two-component developer (that may be hereinafter referred to simply as “developer”) containing a toner and a carrier in developing the electrostatic latent image. The developing device as just stated, namely a two-component developing device, develops the electrostatic latent image in a manner that the developer stored in a developer container is borne on a developer bearing member and thereby delivered to a development area facing the photoreceptor, in which area the toner contained in the developer is then supplied to the photoreceptor. In the developing device, the developer is borne on the developer bearing member with the aid of magnetic force of a magnet roller which is built in the developer bearing member.
In the two-component developing device, the toner is consumed in the image formation while the carrier is repeatedly used for a certain period of time. The carrier deteriorates under various stresses during the repeated use. The stresses include, for example, changes in an environment represented by a temperature and humidity under which the image forming apparatus is provided; heat generated by various parts in the image forming apparatus during a long-lasting operation of the image forming apparatus; and pressure caused by rapid movement of the developer into a narrow gap which is left with a layer thickness-regulating member for regulating a layer thickness of the developer. At all times, the toner is electrically attached to a surface of the carrier owing to an image force thereof. The above-mentioned stresses induce a phenomenon called spent that the toner on the surface of the carrier remains thereon and will eventually cover the surface of the carrier over time. There also appears a phenomenon that a coating material covering the surface of the carrier falls away. The developer therefore has lower flowability, resulting in a decrease in an amount of the developer which is delivered by the developer bearing member. This moreover causes a decrease in a thickness of toner layer regulated by the layer thickness-regulating member, therefore leading to a decrease in an amount of the developer which is used for developing the electrostatic latent image on the photoreceptor. As a result, an image having lower density will be formed.
The carrier and the toner are charged by friction therebetween, and charge amounts thereof are maintained by respective insulation properties inherent in both of the carrier and the toner. The charge amount of the toner is, however, influenced largely by environmental changes. For example, under a high-temperature and high-humidity condition, the charge amount of the developer becomes smaller, which causes problems such as occurrence of toner spattering and growing toner consumption.
There has been disclosed the following developing device of conventional art for solving the problems as above. In the developing device of conventional art, an electric magnet is disposed upstream of a magnetic blade as viewed in a direction where the developer is delivered, and an amount of electricity applied to the electric magnet is controlled in accordance with an atmosphere, to thereby change the magnetic force induced on the magnetic blade and thus control the thickness of the toner layer so as not to generate image defects such as uneven density and image fogging even when the atmosphere varies (refer to Japanese Unexamined Patent Publication JP-A 2005-106874, for example).
In a developing device described in Japanese Unexamined Patent Publication JP-A 2005-134774, a magnet member having at least two magnetic poles of an S-pole and an N-pole is disposed upstream of a layer thickness-regulating member as viewed in a direction where a rotary sleeve rotates so that the magnet member faces the rotary sleeve and so that one magnetic pole of the magnet member is the same as one magnetic pole of a fixed magnet roller during a developing process, whereby an amount of a toner flowing toward the layer thickness-regulating member is regulated.
During a non-developing process, the magnet member is driven to rotate in a predetermined rotational direction to thereby change a magnetic field distribution which is formed between the magnet member and the fixed magnet roller by rotation of the magnet member, so that a toner aggregate is vibrated to be dissolved. This prevents image defects which result from the toner aggregate in a gap between the rotary sleeve and the layer thickness-regulating member.
In the developing device described in JP-A 2005-106874, the electric magnet has a coil which easily catches the toner and thus causes a problem that the developer is hard to be taken out when replacing the developer. Further, the coil is constantly in contact with the developer at high pressure, which accelerates deterioration of the coil and thus requires frequent replacement of the coils. It is thus difficult to maintain the performance. Furthermore, the electric magnet is disposed so as to extend along the developer bearing member, posing such problems that the electric magnet has a larger size and a complex structure which increase in cost.
Further, in the developing device described in JP-A 2005-134774, the amount of the toner flowing toward the layer thickness-regulating member is regulated in a manner that the magnet member is disposed and made to rotate. The configuration as above has an effect of dissolving the toner aggregate, but does not focus on a property of agitating a toner newly-supplied into the developer tank, therefore causing a problem that image defects appear as the property decreases in the case of long-lasting continuous printing with high print ratio.